1527
formation of additional higher order complexes in Cu2+. These facts suggest that the transition state in the Cu(I1)-promoted hydrolysis of I may be formulated as either 1 or 2. 0
I
-0-p-o-
I
H 1
In compound I the juxtaposition of an excellent second binding site (imidazolyl nitrogen) promotes Cu2+interaction with the ether rather than phosphoryl oxygens. The planar relationship between the chelating ligands, furthermore, favors formation of a typical Cu2+ square planar complex. The overall result is to confer hydrolytic ability on the normally nonreactive phosphate monoester dianion. Stephen J. Benkovic,* Leonard K. Dunikoski, Jr. Department of Chemistry, The Pennsylcania State University Unicersity Park, Pennsylvania 16802 Receiced December 28, 1970
2
In 1, the metal ion acts as a more effective acid catalyst than a proton, lowering the pKa of the leaving group so that facile hydrolysis of the dianion-generally encountered only with leaving groups of pK, < 7-may be observed. In 2 the Cu2+ may also serve to induce strain in the P-0 bond and/or partially neutralize charge on the phosphate, leading to a nucleophilic displacement by solvent on phosphorus. It also is noted that hydrolysis of 2 via nucleophilic attack by water is kinetically indistinguishable from hydroxide attack on the monoanion of I. These second possibilities recall the rate accelerations of 105-107 encountered in strained five-membered cyclic phosphates and the 3200-fold enhancement in Cu2+-catalyzed glycine ethyl ester hydrolysis, presumably owing to an increased susceptibility to nucleophilic attack as a result of chelation. Solvolysis of I in mixed methanol-water solvents in the presence of Cu2+ displayed an identical methyl phosphate-inorganic phosphate product ratio with that observed in the uncatalyzed reaction, indicating no change of metaphosphate activity. This finding also rules against a possible phosphoramidate intermediate. I f 2 or its kinetic equivalent represents the reaction pathway then one might have predicted a greater amount of methyl phosphate product owing to the more favorable susceptibility of 2 to nucleophilic attack on phosphorus. Further it might be expected that hydrolysis via 2 may proceed through preequilibrium formation of a pentacovalent phosphorus structure which would result in exchange between solvent and phosphoryl oxygens. When the hydrolysis of I in the presence of Cu2+ was conducted in H2180-enriched water, no evidence of incorporation was observed. Thus it appears reasonable to argue that the Cu2+ ion in this reaction is acting simply as an acid catalyst. Analysis of the structure-reactivity relationship for phosphate esters4 indicates that in order to account for the rate acceleration observed in this reaction, Cu2+ would have to lower the pK, of the leaving group to pKa = 2 , a change of 6 pK, units. The results of this work indicate two important factors which should be considered in the design of future model systems: the requirement for a rigid structural geometry, and the binding of the metal ion to the alcohol leaving group. Compounds such as phenyl phosphate, which have no metal ion binding site other than the phosphate moiety, may show a retarded hydrolytic rate in the presence of metal ions; this is presumably due to binding of the phosphoryl oxygen, making metaphosphate expulsion less likely.
Photolysis of Diazomethane in Tetrachloromethane. An Abstraction Reaction of Singlet Methylene Sir: In this communication we present evidence based on nuclear polarization studies that the singlet state of methylene is involved in insertion reactions into, and abstraction reactions from, tetrachloromethane. Urry and Eiszner discovered the remarkable photoreaction of diazomethane in tetrachloromethane, which affords pentaerythrityl tetrachloride (4).la This product arises from the reaction of one molecule of tetrachloromethane with four molecules of diazomethane, despite the large excess of tetrachloromethane. The high percentage yield of 4, the high quantum yield of the consumption of diazomethane,lb and analogous reactions of diazomethane with other polyhalomethaneslcSd were accommodated in the free-radical chain mechanism outlined in Scheme I.lb The important features of this mechanism are (a) that photochemically generated methylene2 abstracts a chlorine atom from the solvent to start the chain, (b) that intermediate a-chloro radicals, Le., A’, B’, and C’, are stable to the extent that they do not readily abstract chlorine atoms from the solvent, but still readily induce the decomposition of diazomethane (Scheme I, diagonal arrows), (c) that the intramolecular chlorine migration in &y-chloro radical intermediates A, B, and C (Scheme I, horizontal arrows) is considerably faster than any intermolecular reaction of these radicals, and (d) that D, an unstable primary radical without a chlorine atom in the /3 position, abstracts a chlorine atom from the abundant solvent. A more detailed insight into this reaction can be expected from utilizing the recent o b ~ e r v a t i o n ~that ,~ radical reactions may afford products with polarized nuclear spins (CIDNP effect), whose nmr spectra exhibit signals in emission (E) or enhanced absorption (A). This technique may allow one to detect products (1) (a) W. H. Urry and J. R. Eiszner, J . Amer. Chem. Soc., 73, 2977 (1951); (b) W. H. Urry, J. R. Eiszner, and J. W. Wilt, ibid., 79, 918 (1957); (c) W. H. Urry and N. Bilow, ibid., 86, 1815 (1964); (d) N. Bilow, Ph.D. Dissertation, University of Chicago, 1956. (2) Following a suggestion in the literature3 the term “methylene” is
employed to indicate that the divalent carbon intermediate, :CH2, has a triplet ground state.4 (3) (a) R. M. Etter, H. S . Skovronek, and P. S. Skell, J . Amer. Chem. SOC.,81,1008 (1959); (b) A. M. Trozzolo, R. W. Murray, and E. Wasserman, ibid., 84, 4990 (1962). (4) Cf. (a) R. A. Bernheim, W. H. Bernhard, P. S . Wang, L. S . Wood, and P. S. Skell, J . Chem. Phys., 53, 1280 (1970); (b) E. Wasserman, W. A . Yager, and V. J. Kuck, Chem. Phys. Lett., 7, 409 (1970). (5) (a) J. Bargon, H. Fischer, and U. Johnson, 2.Nuturforsch. A , 22, 1551 (1967); (b) J. Bargon and H. Fischer, ibid., A , 22, 1556 (1967). (6) (a) H. R. Ward and R. G. Lawler, J . Amer. Chem. Soc., 89, 5518 (1967); (b) R. G. Lawler, ibid., 89, 5519 (1967).
Communications to the Editor
1528
l
~
I
~
---t
H
Figure 1. Fast-sweep 60-MHz spectra of solutions of diazomethane in carbon tetrachloride, before (b), during (a and c), and after ultraviolet irradiation. The solution used for recording trace (a) also contained 10% benzophenone. A frequency scale is given below trace d ; the distance between two markers represents 10.6 Hz.
that are formed in minor amounts or are unstable under the conditions of reaction or work-up. Scheme I
I
: CHz
*CH2CI
c1
CI
I
CI-C-CH2*
I
Cl
A
/
I I
Cl
B
A
/
I I
---+
-cc1,
A
I I
CI-C-CHzCI
Cl
Cl
A'
1
I . C-CH2CI I CHZCI
C1 cc1, ---+
-h,
6'
I . C-CH2C1 I
I
CI-C-CH2CI
I
CHzCl
2
CH2Cl
CI CICH2-C-CH2.
CI cc4
CI
CI CICHz-C-CH2.
I . C-CH2C1 I
CHzC1 cc1
-&:+
I
C 1 -C-CHzC1
I
CH2CI
3 CHzC1
I
CICHz-C-CHz~
I
spectrometer, exhibits four nmr signals (Figure IC) which were identified by their chemical shifts as due to lY1,1,2-tetrachloroethane(1, strong E), 1,2,2,3-tetrachloropropane (2, weak A), 1,2,3-trichloro-2-(chloromethy1)propane (3, very weak E), and 4 (weak A). Only the major product of this reaction, 4, was formed in sufficient yield to allow its detection after the irradiation was discontinued (Figure Id).' An attractive mechanism for the formation of 1 is the recombination of a geminate radical pair, chloromethyl-trichloromethyl. The formation of this pair, and subsequent escape from its solvent cage, initiates the free-radical chain leading to 4. It is only reasonable that some of these pairs recombine in their original cages (two-step insertion). Our assignment of a spin multiplicity to the reactive intermediate, methylene, is based on the polarization of product 1. Therefore, it is necessary to establish that the enhanced signal of 1 represents more than an insignificant percentage of that product. An approximate enhancement factor for 1 can be estimated crudely 1 is recorded in approxias ~ follows. The signal of ] ~ mately ten times the starting intensity of the diazomethane signal when less than one-third of that material is decomposed. The yield of 1 is
